== Do control characters validate?

This is a C-d:  right there.

Here is it wrapped up in ((code:  and)) [[plain  <-- there|]].

This may or may not validated.

No, it's not. To quote the validator on excluded characters:
> (0 to 31 inclusive and 127 to 159 inclusive)

(but we want to skip tab & newline.) Steal badchars from the
comments module I think.

See http://www.w3.org/MarkUp/html3/latin1.html
and http://ppewww.ph.gla.ac.uk/%7Eflavell/charset/

Also in a <pre> block, for completeness:

 This should be C-d right here:  <-- there.
 Multiple in a <pre>:  .
 And this is done.

Okay, this is a test of several:  done.